A signal processing apparatus has been known which controls the dynamic range (ratio of the maximum value to the minimum value of a volume level) of various types of audio contents such as a music signal downloaded via the Internet, an audio signal packaged in a Blu-ray (registered trademark) Disk (BD), Digital Versatile Disk (DVD), or Compact Disk (CD), and a radio or TV broadcast audio signal. Carrying out the dynamic range control enables raising a low volume level of a sound signal or reversely lowering a high volume level of the sound signal, thereby being able to keep the volume level of the various contents at a uniform level.
For example, an AGC (automatic gain control) apparatus disclosed in Patent Document 1 comprises a first smoothing unit that smoothes an input acoustic signal at a time constant T1 and a second smoothing unit that smoothes it at a time constant T2 (>T1), thereby generating output signals with two volume levels. Then, it controls the dynamic range at an attack in accordance with the volume level of the output signal smoothed at the time constant T1, and controls the dynamic range at a release in accordance with the volume level of the output signal smoothed at the time constant T2. Here, the term “at an attack” refers to a case where the level varies from a low volume level to a high volume level, and the term “at a release” refers to a case where the level varies from a high volume level to a low volume level. Switching between the two time constants T1 and T2, the AGC apparatus can control the dynamic range while suppressing the production of unnatural sounds at the volume level change.